Composition and method

ABSTRACT

A method includes contacting a transition metal oxide, a sintering additive, and a grain growth inhibitor additive to form a mixture. The transition metal oxide include particles that have an average diameter less than about 1 micrometer and treating the mixture to a temperature profile that is sufficiently high that a sintered mass is formed from the mixture. The thermal profile is less than about 1050 degrees Celsius.

RELATED APPLICATIONS

This application is a non-provisional application that claims priorityto provisional U.S. Pat. application Ser. No. 60/991,871, filed Dec. 03,2007; the disclosure of which is hereby incorporated by reference.

BACKGROUND

1. Technical Field

The invention includes embodiments that relate to a composition for useas a surge protector and/or varistor. The invention includes embodimentsthat relate to a method of making and/or using the composition, orderived device.

2. Discussion of Art

A varistor is an electronic component with a non-ohmic current-voltagecharacteristic. Varistors may protect circuits against excessivetransient voltages by incorporating them into the circuit in such a waythat, when triggered, they will shunt the current created by the highvoltage away from the sensitive components. A varistor may be known asVoltage Dependent Resistor or VDR.

A type of varistor is the Metal Oxide Varistor (MOV). This contains aceramic mass of zinc oxide grains, in a matrix of other metal oxides(such as small amounts of bismuth, cobalt, manganese) sandwiched betweentwo metal plates (the electrodes). The boundary between each grain andits neighbour forms a diode junction, which allows current to flow inonly one direction. The mass of randomly oriented grains is electricallyequivalent to a network of back-to-back diode pairs, each pair inparallel with many other pairs. When a small or moderate voltage isapplied across the electrodes, only a tiny current flows, caused byreverse leakage through the diode junctions. When a large voltage isapplied, the diode junctions break down because of the avalanche effect,and a large current flows. The result of this behaviour is a highlynonlinear current-voltage characteristic, in which the MOV has a highresistance at low voltages and a low resistance at high voltages.

A varistor remains non-conductive as a shunt mode device during normaloperation when voltage remains well below its “clamping voltage”. If atransient pulse (often measured in joules) is too high, the device maymelt, burn, vaporize, or otherwise be damaged or destroyed. Thisunacceptable (catastrophic) failure occurs when “Absolute MaximumRatings” are exceeded. Varistor degradation is defined using curves thatrelate current, time, and number of transient pulses. A varistor fullydegrades when its “clamping voltage” has changed by 10 percent. Afully-degraded varistor may remain functional, having no catastrophicfailure, and may not be visually damaged.

It may be desirable to have a method that differs from those methodscurrently available to provide a composition or article with propertiesand characteristics that differ from those properties of currentlyavailable compositions and articles.

BRIEF DESCRIPTION

In one embodiment, a method is provided that includes contacting atransition metal oxide, a sintering additive, and a grain growthinhibitor additive to form a mixture, and treating the mixture to atemperature profile that is sufficiently high that a sintered mass isformed from the mixture, and the thermal profile is less than about 1050degrees Celsius. The transition metal oxide comprises particles thathave an average diameter less than about 1 micrometer.

In one embodiment, a method provides a sintered mass having a pluralityof cores and a grain boundary layer disposed between each of theplurality of cores. The core includes a transition metal oxide, andgrain boundary layer includes a sintering additive and a grain growthinhibitor additive.

In one embodiment, a method includes contacting a transition metaloxide, and a sintering additive and calcining the transition metal oxideand the sintering additive together, wherein the calcining comprisesheating to a temperature of about 400 degress Celsius to provide acalcined mass. The calcined mass is contacted with a grain growthinhibitor additive to form a mixture and treating the mixture to atemperature profile that is sufficiently high that a sintered mass isformed from the mixture, and the thermal profile is less than about 1050degrees Celsius is provided. The transition metal oxide comprisesparticles that have an average diameter less than about 1 micrometer.

In one embodiment, a method includes contacting a transition metaloxide, a sintering additive, and a grain growth inhibitor additive toform a mixture, and treating the mixture to a temperature profile thatis sufficiently high that a sintered mass is formed from the mixture,and the thermal profile is less than about 1050 degrees Celsius. Thesintered mass can be contacted with an electrically conductive metal toform a electrical connection therebetween.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a block diagram of a method to make a sintered mass inaccordance with one embodiment of the invention.

FIG. 2 shows a block diagram of a method to make a sintered mass inaccordance with one embodiment of the invention.

FIG. 3 shows a graph of the electric field versus the current density(current voltage graph) for a composition in accordance with oneembodiment of the invention and a comparative sample.

FIG. 4 shows SEM micrographs of a composition in accordance with anembodiment and a control blank.

FIG. 5 shows SEM micrographs of a composition in accordance with oneembodiment of the invention.

DETAILED DESCRIPTION

The invention includes embodiments that relate to a composition for useas a surge protector and/or varistor. The invention includes embodimentsthat relate to a method of making and/or using the composition, or thederived device.

As used herein, the term sintering is a method for making objects fromparticles or powder by heating the material (below its melting point)until its particles adhere to each other. Sintered refers to particlesor powder that has undergone a sintering process. A sintered mass refersto the formed shape that is the result of the sintering of powders orparticulate. In the sintered mass, formerly discrete particles or powdergrains retain a core, and the interstitial area from one core to anothercore is at least partially filled with a grain boundary layer thatseparates the cores.

In one embodiment, a composition includes a sintered mass. The sinteredmass includes a plurality of particle cores and a grain boundary layerdisposed between each of the plurality of particle cores. Each of thecores may include a transition metal oxide. The grain boundary layerincludes a sintering additive, a grain boundary additive, and/or abreakdown voltage additive.

In one embodiment, the particle core may include a transition metal. Inone embodiment, the transition metal may be a transition metal oxide.Examples of transition metal oxides include but are not limited to zincoxide, tin oxide, and titanium oxide. In one embodiment, the transitionmetal oxide includes a zinc oxide. The amount of the transition metaloxide, by weight, may be greater than about 80 percent based on thetotal weight of the sintered mass. In one embodiment, the amount may bein a range of from about 80 weight percent to about 85 weight percent,from about 85 weight percent to about 90 weight percent, or from about90 weight percent to about 95 weight percent, or from about 95 weightpercent to about 98 weight percent based on the total weight of thesintered mass.

In one embodiment, the grain boundary layer is disposed between each ofthe plurality of the cores. The grain boundary layer includes asintering additive. In one embodiment, the sintering additive mayinclude one or more of aluminum, lithium, antimony, bismuth, cobalt,chromium, manganese, nickel, magnesium, or silicon. The sinteringadditive may include a combination of two or more of the foregoing. Inone embodiment, the sintering additive includes one or more of SiO₂,Mn₂O₃, NiO, MnO₂, or MnCO₃. In one embodiment, the sintering additivemay include one or more of Li₂CO₃, or LiBiO₃. In one embodiment, thesintering additive may include only one of the foregoing. The selectionof the sintering additive may be based on one or more factors as thesintering additives differ in efficacy and effect. Such factors mayinclude the desired sintering temperature, the sintering pressure, thematerial performance, and the desired grain characteristics.

The sintering additive may be present in an amount that is less thanabout 15 percent by weight, based on the total weight of the sinteredmass. In one embodiment, the sintering additive amount may be in a rangeof from about 15 percent to about 12 percent, from about 12 percent toabout 10 percent, from about 10 percent to about 8 percent, from about 8percent to about 4 percent, from about 4 percent to about 2 percent,from about 2 percent to about 0.5 percent, from about 0.5 percent toabout 0.3 percent, or from about 0.3 percent to about 0.1 percent, orfrom about 0.1 percent to about 0.03 percent.

In one embodiment, the grain boundary includes a grain growth inhibitoradditive. In one embodiment, the grain growth inhibitor additive mayinclude one or more of Sb₂O₃, CaO, Al₂O₃, MgO, or Fe₂O₃. In oneembodiment, the grain growth inhibitor may consist essentially of onlyone of the foregoing. The selection of the grain growth inhibitoradditive may be based on one or more factors as the grain growthinhibitor additive differ in efficacy and effect. Such factors mayinclude the desired sintering temperature, the sintering pressure, thematerial performance, and the desired grain characteristics. In oneembodiment, the grain growth inhibitor additive may inhibit grain growthto maintain relatively smaller grains. The grain growth inhibitoradditive may control the grain size distribution, as well. In oneembodiment, the grain growth inhibitor additive may be present in anamount in a range of from about 0.1 weight percent to about 0.5 weightpercent, from about 0.5 weight percent to about 1.5 weight percent, orfrom about 1.5 weight percent to about 3 weight percent.

In one embodiment, the grain growth inhibitor additive may include acombination of two or more of the foregoing. In one embodiment, thegrain growth inhibitor additive may be present in the sintered mass inan amount, by weight, that is less than about 10 percent based on thetotal weight of the sintered mass. In one embodiment, the grain growthinhibitor additive amount may be in a range of from about 10 weightpercent to about 8 weight percent, from about 8 weight percent to about6 weight percent, 6 weight percent to about 4 weight percent, from about4 weight percent to about 2 weight percent, from about 2 weight percentto about 1 weight percent, from about 1 weight percent to about 0.5weight percent, from about 0.5 weight percent to about 0.1 weightpercent, or less than about 0.1 weight percent.

In one embodiment, the composition may further include a grain boundaryadditive. In one embodiment, the grain boundary additive includes abreakdown voltage additive. In one embodiment, the grain boundaryadditive may enhance the grain boundary barrier. In one embodiment, thegrain boundary additive may include one or more of Co₃O₄, Co₂O₃, Cr₂O₃,Bi₂O₃, Pr₂O₃, NiO, or SnO₂. In one embodiment, the grain boundaryadditive consists essentially of only one of the foregoing. Theselection of the grain boundary additive may be based on one or morefactors as the grain boundary additive differ in efficacy and effect.Such factors may include the desired sintering temperature, thesintering pressure, the material performance, and the desired graincharacteristics. The grain boundary additive may be present in an amountless than about 1 weight percent. In one embodiment, the grain boundaryadditive may be present in an amount in a range of from about 0.01weight percent to about 0.5 weight percent, from about 0.5 weightpercent to about 0.75 weight percent, or from about 0.75 weight percentto about 1 weight percent. In one embodiment, the composition is free ofCo₂O₃. In another embodiment, the amount of Co₂O₃ is less than about0.05 weight percent.

In one embodiment, the additive may include a combination of two or moreof the foregoing. In one embodiment, the grain boundary additive may bepresent in the sintered mass in an amount, by weight, that is less thanabout 10 percent based on the total weight of the sintered mass. In oneembodiment, the grain boundary additive is present in an amount in arange of from about 10 weight percent to about 8 weight percent, fromabout 8 weight percent to about 6 weight percent, from about 6 weightpercent to about 4 weight percent, from about 4 weight percent to about2 weight percent, from about 2 weight percent to about 1 weight percent,from about 1 weight percent to about 0.5 weight percent, from about 0.5weight percent to about 0.1 weight percent, or less than about 0.1weight percent.

In one embodiment, the average distance from one core to an adjacentcore in the plurality of cores is less than about 1 micrometer. In oneembodiment, the average distance may be in a range of from about 1micrometer to about 0.8 micrometers, or from about 0.8 micrometers toabout 0.5 micrometers. In another embodiment, the average distance maybe in a range of from about 500 nanometers to about 400 nanometers, fromabout 400 nanometers to about 300 nanometers, from about 300 nanometersto about 250 nanometers, from about 250 nanometers to about 200nanometers, from about 200 nanometers to about 150 nanometers, fromabout 150 nanometers to about 100 nanometers, from about 100 nanometersto about 50 nanometers, or less than about 50 nanometers.

In one embodiment, the average diameter of the core in the plurality ofcores is less than about 1 micrometer. In one embodiment, the averagediameter may be in a range of from about 1 micrometer to about 0.8micrometers, or from about 0.8 micrometers to about 0.5 micrometers. Inanother embodiment, the average diameter may be in a range of from about500 nanometers to about 400 nanometers, from about 400 nanometers toabout 300 nanometers, from about 300 nanometers to about 250 nanometers,from about 250 nanometers to about 200 nanometers, from about 200nanometers to about 150 nanometers, from about 150 nanometers to about100 nanometers, from about 100 nanometers to about 50 nanometers, orless than about 50 nanometers.

The micro-structure or nano-structure of the composition may beexpressed in terms of an average distance from one core to an adjacentcore in the sintered mass. The average distance from one core to anadjacent core in the sintered mass may be less than 5 micrometers. Inone embodiment, the average distance may be in a range of from about 1micrometer to about 0.8 micrometers, or from about 0.8 micrometers toabout 0.5 micrometers. In another embodiment, the average distance maybe in a range of from about 500 nanometers to about 400 nanometers, fromabout 400 nanometers to about 300 nanometers, from about 300 nanometersto about 250 nanometers, from about 250 nanometers to about 200nanometers, from about 200 nanometers to about 150 nanometers, fromabout 150 nanometers to about 100 nanometers, from about 100 nanometersto about 50 nanometers, or less than about 50 nanometers. An exemplarycore-to-core average distance may be in a range of from about 35nanometers to about 75 nanometers.

The distance of one core to another core, coupled with the core size,may affect the average thickness of the grain boundary layer. In oneembodiment, the average thickness of the grain boundary layer may beless than about 1 micrometer. In another embodiment, the averagethickness may be in a range of from about 1 micrometer to about 0.8micrometers, or from about 0.8 micrometers to about 0.5 micrometers. Inyet another embodiment, the average thickness may be in a range of fromabout 500 nanometers to about 400 nanometers, from about 400 nanometersto about 300 nanometers, from about 300 nanometers to about 250nanometers, from about 250 nanometers to about 100 nanometers, fromabout 100 nanometers to about 50 nanometers, from about 50 nanometers toabout 35 nanometers, from about 35 nanometers to about 20 nanometers, orless than about 20 nanometers.

The grain boundary layer thickness, may be expressed as a mean value innanometers. The mean value for the grain boundary layer may be less thanabout 50 nanometers. In one embodiment, the mean value may be in a rangeof from about 50 nanometers to about 10 nanometers, from about 10nanometers to about 1 nanometer, or from about 1 nanometer to about 0.1nanometers.

In addition to such factors as the uniformity of core diameters, theuniformity of distribution of materials, and the uniformity of the grainboundary layer, the average distance of the cores from one to anothermay affect the performance, properties and characteristics of thevaristor device made therefrom. Particularly, the diode junctionperformance, and the number of diode junctions per unit volume, may flowdirectly from the core spacing parameter.

In one embodiment, the sintered mass may have a dielectric strength orbreakdown field of greater than about 0.5 kV/mm. In one embodiment, thedielectric strength or breakdown field is in a range of from about 0.5kV/mm to about 1 kV/mm, from about 1 kV/mm to about 1.5 kV/mm, fromabout 1.5 kV/mm to about 2 kV/mm, from about 2 kV/mm to about 2.5 kV/mm,from about 2.5 kV/mm to about 2.8 kV/mm, or greater than about 2.8kV/mm. In one embodiment, the sintered mass may have a non-linearitycoefficient (α) of greater than 25. In one embodiment, the non-linearitycoefficient (α) may be in a range of from about 25 to about 50, fromabout 50 to about 75, from about 75 to about 100, from about 100 toabout 125, from about 125 to about 140, or greater than about 140.

The thermal profile may play a role in the melt temperature of theelectrode of the MOV device. If the thermal profile is higher than theelectrode melt temperature, then the electrode may be melted, damaged ordestroyed. A higher thermal excursion during manufacture or sinter maythen require an electrode with a corresponding melt temperature suitablefor use after exposure to that temperature. Lower temperature capableelectrode materials may be economically desirable, if the otherperformance parameters are correct. In addition, if the thermal profileshows a temperature excursion too high, the micro-structure ornano-structure may change and the sintered particles may melt and flowtogether rather than remain as a sintered mass. This may need to bebalanced, as at least some heat is needed to get the particles to sinterin the first instance.

In one embodiment, a sintered mass may be produced by mixing atransition metal oxide, a sintering additive, and a grain boundaryadditive under defined conditions to form a mixture. The mixture can betreated to a determined temperature profile. In one embodiment, thetemperature profile includes exposure to a sinter temperature of lessthan about 1050 degrees Celsius. The composition may have a thermalprofile also known as thermal history that may include exposure to asintering temperature of not greater than about 1050 degrees Celsius. Inone embodiment, the thermal profile includes exposure to a sintertemperature in a range of from about 1050 degrees Celsius to about 1000degrees Celsius, from about 1000 degrees Celsius to about 950 degreesCelsius, from about 950 degrees Celsius to about 900 degrees Celsius,from about 900 degrees Celsius to about 875 degrees Celsius, or fromabout 875 degrees Celsius to about 850 degrees Celsius.

In one embodiment, the method includes calcining the transition metaloxide and the sintering additive together before forming the mixture.Calcining of the transition metal oxide and the sintering additiveincludes heating to a temperature that is greater than about 400 degreesCelsius. In one embodiment, calcining includes exposure of the mixtureto a temperature in a range of from about 400 degrees Celsius to about450 degrees Celsius, from about 450 degrees Celsius to about 550 degreesCelsius, from about 550 degrees Celsius to about 600 degrees Celsius,from about 600 degrees Celsius to about 650 degrees Celsius, or fromabout 650 degrees Celsius to about 800 degrees Celsius.

In one embodiment, a method includes contacting a transition metal oxidewith a sintering additive to form a premix, wherein the transition metaloxide comprises particles that have an average diameter less than about1 micrometer. The premix may be calcined. The clacining includes heatingto a temperature of about 400 degress Celsius to provide a calcinedmass. The calcined mass may be contacted with a grain growth inhibitoradditive to form a mixture. The mixture is sintered at a temperatureprofile that is sufficiently high that a sintered mass is formed fromthe mixture, and the thermal profile is less than about 1050 degreesCelsius.

In one embodiment, the method includes contacting the sintered mass withan electrically conductive material to form an electrical connectiontherebetween. The electrically conductive material electrode may beformed by pressing, heating, melt flowing, casting, printing,metallizing/etching, and the like. Mechanical methods of attachment maybe available in some embodiments. Alternatively, a precursor materialmay be disposed on the sintered mass and converted into an electricallyconductive material. In one embodiment, the electrically conductivematerials may include one or more of platinum, palladium, copper,silver, tin, aluminum, iron, carbon, nickel, antimony, chromium, orgold. Alloys of the foregoing are also suitable based on applicationspecific parameters (such as brass or Ni—Sn). In one embodiment, theelectrically conductive material consists essentially of silver. In oneembodiment, the electrically conductive material includes carbon, andthe carbon is amorphous or structured (such as in a nanotube ornanowire).

A block diagram 10 of one embodiment of a method is provided in FIG. 1.The transition metal oxide and the sintering additive and the graingrowth inhibitor additive are provided or obtained 12. A solvent wetsthe transition metal oxide and the sintering additive and the graingrowth inhibitor additive to form a wet mixture 14. The wet mixture isdried to provide a dry mixture 16. The dry mixture is powdered toprovide a powdered dry mixture 18. The powdered dry mixture is calcinedto form a calcined mass 20. The calcined mass is compacted to form apowdered mass 22. The powdered mass is sintered at a temperature that isless than about 1050 degrees Celsius to provide the sintered mass 24.

A block diagram 30 of one embodiment of a method is provided in FIG. 2.A transition metal oxide is provided 32. A solvent wets the transitionmetal oxide to form a wet slurry 34. The wet slurry is dried to providea dry mixture 36. The dry mixture is powdered to provide a powdered drymixture 38. The powdered dry mixture is calcined to form a calcined mass40. The calcined mass is contacted with the sintering additive and thegrain growth inhibitor additive 42 to form a calcined mixture. A solventwets the calcined mixture to form a wet calcined mixture 44. The wetcalcined mixture is dried to provide a dry calcined mixture 46. The drymass is powdered to provide a powdered mass 48. The powdered mass iscalcined to provide a calcined powdered mass 50. The calcined powderedmass is sintered at a temperature that is less than 1050 degrees Celsiusto provide the sintered mass 52.

In one embodiment, the composition includes a sintered reaction productof transition metal oxide particles that have an average diameter thatis less than about 1 micrometer; and sintering additive particles havingan average diameter that is less than about 1 micrometer. The graingrowth inhibitor additive particles may have an average diameter that isless than about 1 micrometer. Due to the change in available surfacearea, and packing tendencies, particles of different sizes may formsintered masses having differing properties and characteristics.

In one embodiment, the composition includes a sintered mass of particlesthat may include a transition metal oxide, a sintering additive, and agrain growth inhibitor additive. The sintered mass may have a densitythat is greater than 98 percent of theoretical density for a compositioncomprising the transition metal oxide.

In one embodiment, the composition includes sintered particles thatinclude a transition metal oxide, a sintering additive, and a graingrowth inhibitor additive and defining grains. The grains may have grainboundaries that define the grains to have an average grain size of lessthan about 0.8 micrometers. In one embodiment, the method includescontacting a transition metal oxide, a sintering additive, and a graingrowth inhibitor additive to form a mixture. The mixture may be treatedto a temperature profile. In one embodiment, the temperature profileincludes exposure to a sinter temperature of less than about 1050degrees Celsius.

EXAMPLES

The following examples illustrate methods and embodiments in accordancewith the invention, and as such do not limit the claims. Unlessspecified otherwise, all ingredients may be commercially available fromsuch common chemical suppliers as Alpha Aesar, Inc. (Ward Hill, Mass.),Sigma Aldrich (St. Louis, Mo.), Spectrum Chemical Mfg. Corp. (Gardena,Calif.), and the like.

Examples 1 through 3 are prepared by mixing, calcining, ball milling,and sintering. The sintering is performed in a Uniaxial Press to make apuck for each sample that is about 1 inch in diameter. The variouscomponents and the weight percent for each of the components forexamples 1 to 3 are given in Table 1.

TABLE 1 Composition (Weight Sample Sample Comparative Comparativepercent) Sample 1 2 3 Sample 1 Sample 2 ZnO 94 85.5 94.69 83.39 92.21Bi₂O₃ 0.5 2 3 2.12 1.40 Sb₂O₃ 1 3 1.5 6.34 3.75 Al₂O₃ — 2 0.01 0.04 —SiO₂ 2 3 — 0.43 0.07 Cr₂O₃ 0.5 — — — 1.02 MnO — — — 0.9 0.4 Mn₂O₃ 0.5 —0.1 — — MgO — 2 — — — Fe₂O₃ — — — 0.01 0.04 Co₂O₃ — — — 1.13 1.17 Co₃O₄0.5 2.5 0.5 — — NiO 1 — 0.2 1.27 — SnO₂ — — — — 0.93

Example 1

A mixture is formed from zinc oxide, and additives selected from cobalt,antimony, nickel, and chromium oxide nanopowders with bismuth, silicon,manganese oxide nanopowders in a ratio given in Table 1. The zinc oxideis commercially obtainable from Horsehead Corporation, (Monaca, Pa.).The additives are commercially obtainable from Nanostructured andAmorphous Materials Inc. (Houston, Tex.).

The materials form a mixture in a mixed oxide wet process. The mixtureis milled in a ball mill for about 6 hours in a ratiomaterials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry isdried at 100 degrees Celsius. The dried powder is sieved and calcined at550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. Thecalcined powder is then ball milled for about 4 hours. The slurry formedis dried at 100 degrees Celsius and the dried powder is sieved. Thepowder is then pressed into pellets (thickness of about 1.5 millimeters)with a force of about 10000 pounds for about 1 minute. The pellet issintered at temperatures from about 1000 degrees Celsius and 1050degrees Celsius. The sintering is done in two different profilesincluding one and two steps in a Uniaxial Press for about 2 hours. Thefirst profile is carried out at about at 1050 degrees Celsius at aheating rate of about 5 degrees Celsius per minute for about 2 hours andis allowed to cool. The second profile is carried out at about at 1000degrees Celsius at a rate of about 10 degrees Celsius per minute forabout 0.1 hours. Following this a second step sintering at a temperatureof about 925 degrees Celsius to 975 degrees Celsius at a heating rate ofabout 10 degrees Celsius per minute is carried out for about 2 hours.The resultant product is Sample 1, which has the compositionaldistribution as indicated in Table 1.

Example 2

A mixture is formed from zinc oxide, and additives selected from oxidenanopowders cobalt, and antimony, with nanopowder oxides of bismuth,silicon, aluminum and magnesium in a ratio given in Table 1.

The materials are mixed using a mixed Oxide Wet Process. The mixture ismilled in a ball mill for about 6 hours in a ratiomaterials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry isdried at 100 degrees Celsius. The dried powder is sieved and calcined at550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. Thecalcined powder is then ball milled for about 4 hours. The slurry formedis dried at 100 degrees Celsius and the dried powder is sieved. Thepowder is then pressed into pellets (thickness of about 1.5 millimeters)with a force of about 10000 pounds for about 1 minute. The pellet issintered in a Uniaxial Press at different temperatures for about 2 hoursat about 950 degrees Celsius, about 1050 degrees Celsius at a rate ofabout 5 degrees Celsius per minute. The resultant product is Sample 2,which has the compositional distribution indicated in Table 1.

Example 3

A mixture is formed from zinc oxide (from Horsehead Corporation, Monaca,Pa.), and additives selected from powders of cobalt, nickel, andantimony-based materials (from Nanostructured and Amorphous MaterialsInc, Houston, Tex.), and with powders of bismuth, aluminum andmanganese-based materials in amounts as given in Table 1.

The materials are mixed using a mixed oxide wet process. The mixture ismilled in a ball mill for about 6 hours in a ratio of powdermaterials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry isdried at 100 degrees Celsius. The dried powder is sieved and calcined at550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. Thecalcined powder is then ball milled for about 4 hours. The slurry formedis dried at 100 degrees Celsius and the dried powder is sieved. Thepowder is then pressed into pellets (thickness of about 1.5 millimeters)with a force of about 10000 pounds for about 1 minute. The pellet issintered in a pressureless mode at different temperatures for about 2hours at about 850 degrees Celsius, about 900 degrees Celsius, about 950degrees Celsius, about 1000 degrees Celsius and about 1050 degreesCelsius at a rate of about 5 degrees Celsius per minute. The resultantproduct is Sample 3, which has the compositional distribution indicatedin Table 1.

Current—Voltage (I-V) Measurement:

A 10 kiloOhm or 100 MegaOhm resistor is connected in parallel to thevaristor and a voltage is applied. V1, the total voltage on sample andvaristor is measured using a high voltage probe. V2, the voltage on theresistor is measured by a multimeter. V2 is used to calculate thecurrent flowing through the varistor. V1-V2 is the voltage on theSamples 1-6. To measure I-V curve, at low voltage, a 100 MOhm resistoris used until the voltage on it is higher than about 100 Volts. A 10kilo Ohm resistor is used to measure the I-V curve under high voltage(higher than about 100 Volts).

FIG. 3 shows the results of metal oxide varistor materials of Samples1-3 relative to a commercially available metal oxide varistor material.The materials of Samples 1-3 display relatively better breakdownstrength and relatively better nonlinearity compared to ComparativeSample 1. The breakdown fields (electric fields when current density is1 milliAmp per square centimeter) and nonlinearity coefficient αcalculated are summarized in Table 2.

TABLE 2 Breakdown fields and nonlinearity of metal oxide varistormaterials of Samples 1-3 and commercially available metal oxide varistormaterial Sintering Temperature Breakdown (° C.)/ Field NonlinearityComposition Time (Hours) (V/mm) coefficient α Sample 1 1000/2 1343 631050/2 972 138 Sample 2  950/2 2800 40 1000/2 2216 18 Sample 3  850/21710 77  900/2 546 19  950/2 515 77 1000/2 400 42 1050/2 315 79Comparative NA 125 22 Sample 1

Table 2 shows that the metal oxide varistor materials of Samples 1-3perform better after low temperature firing. For example, Sample 3 givesa breakdown field of greater than about 1700 volts per millimeter and agood nonlinearity coefficient (α) of about 77, but still has a lowsintering temperature of 850 degrees Celsius.

Microstructure formation may depend at least in part on the sinteringprofile. Grain size is found to increase at higher sinteringtemperature. FIGS. 4-5 compare the microstructure of a commerciallyavailable metal oxide varistor material Comparative Sample 1 (FIG. 4)with a metal oxide varistor material of Example 3 (FIG. 5). The averagegrain size of the metal oxide varistor materials of Sample 3 (sinteredat 850 degrees Celsius) is less than 1 micrometer; and, this is incomparison to the commercially available metal oxide varistor materialComparative Sample 1 that has a grain size that is greater than 10micrometers. Several phases may coexist in the metal oxide varistormaterials of Samples 1-3. These phases may include the major conductivephase of less than 1 micrometer in size and one or more secondary phaseslocated at the grain boundaries and in the grain boundary layer, whichitself may include various dopants and sintering additives.

Example 4

A mixture is formed from zinc oxide, and additives selected from oxidepowders cobalt, and antimony, with powder oxides of bismuth, silicon,aluminum and chromium in amounts as given in Table 3. Unless otherwiseindicated, the powders are nanoscale and have a narrow sizedistribution.

The materials are mixed using a mixed Oxide Wet Process. The mixture ismilled in a ball mill for about 6 hours in a ratio ofingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurryis dried at 100 degrees Celsius. The dried powder is sieved and calcinedat 550 degrees Celsius for about 2 hours in a THERMOLYNE 1400 furnace.The calcined powder is then ball milled for about 4 hours. The slurryformed is dried at 100 degrees Celsius and the dried powder is sieved.The powder is then pressed into pellets (thickness of about 1.5millimeters) with a force of about 10000 pounds for about 1 minute. Thepellet is sintered in a Uniaxial Press at different temperatures forabout 2 hours at about 950 degrees Celsius, about 1050 degrees Celsiusat a rate of about 5 degrees Celsius per minute. The resultant productis Sample 4, which has the compositional distribution indicated in Table3.

Example 5

A plurality of mixtures are formed from zinc oxide, and additivesselected from cobalt, antimony, nickel, and chromium-based powders withbismuth, silicon, manganese-based powders, each in an amount as given inTable 3. The powders, unless context or language indicates otherwise,are nano-scale and have an average diameter that is less than 100nanometers, and a relatively narrow and mono-modal size distribution.

The materials form a mixture in a mixed oxide wet process. The mixtureis milled in a ball mill for about 6 hours in a ratioingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurryis dried at 100 degrees Celsius. The dried powder is sieved and calcinedat 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace.The calcined powder is then ball milled for about 4 hours. The slurryformed is dried at 100 degrees Celsius and the dried powder is sieved.The powder is then pressed into pellets (thickness of about 1.5millimeters) with a force of about 10,000 pounds for about 1 minute. Thepellet is sintered at temperatures from about 1000 degrees Celsius and1050 degrees Celsius. The sintering is done in two different profilesincluding one and two steps in a Uniaxial Press for about 2 hours. Thefirst profile is carried out at about at 1050 degrees Celsius at aheating rate of about 5 degrees Celsius per minute for about 2 hours andis allowed to cool. The second profile is carried out at about at 1000degrees Celsius at a rate of about 10 degrees Celsius per minute forabout 0.1 hours. Following, a second step includes sintering at atemperature of about 925 degrees Celsius to 975 degrees Celsius. Thesintering is at a heating ramp up rate of about 10 degrees Celsius perminute for about 2 hours. The resultant product is Sample 5, which hasthe compositional distribution as indicated in Table 3.

Example 6

A mixture is formed from zinc oxide, and additives selected from powdersof cobalt, lithium, nickel, and antimony-based materials, with powdersof bismuth, and aluminum-based materials in amounts as given in Table 3.

The materials are mixed using a mixed oxide wet process. The mixture ismilled in a ball mill for about 6 hours in a ratio of powderingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurryis dried at 100 degrees Celsius. The dried powder is sieved and calcinedat 550 degrees Celsius for about 2 hours in a THERMOLYNE 1400 furnace.The calcined powder is then ball milled for about 4 hours. The slurryformed is dried at 100 degrees Celsius and the dried powder is sieved.The powder is then pressed into a plurality of pellets (thickness ofabout 1.5 millimeters) with a force of about 10000 pounds for about 1minute.

The pellets are sintered in a pressureless mode at differenttemperatures for about 2 hours. The temperatures are: about 800 degreesCelsius (sample 6), about 850 degrees Celsius (sample 7), about 900degrees Celsius (sample 8), about 950 degrees Celsius (sample 9), about1000 degrees Celsius (sample 10), and about 1050 degrees Celsius (sample11), each at a rate of about 5 degrees Celsius per minute. Sample 6 issubsequently subjected to each of the other temperature profiles. Theresultant product pellets are represented in Samples 6-11, which havethe compositional distribution indicated in Table 3. Additional samples12 et seq. have the compositional makeup as indicated in Table 3, andare subject to the temperature profile of Sample 7 (850 degrees Celsius)and are prepared in the same manner as the rest of the Samples in thethis example.

Table 3 shows that the metal oxide varistor materials may performrelatively well, displaying a breakdown field of greater than about 1700volts per millimeter and a good nonlinearity coefficient (α) of greaterthan about 75, but still having a relatively low sintering temperature.

TABLE 3 Composition Samples Sample Sample Sample (Wt percent) Sample 4Sample 5 Sample 6 7-11 12 13 14 ZnO 85.5 94 94.69 94.69 95.0 84.0 94.0Bi₂O₃ 2 0.5 3 3 3.5 3 3 Sb₂O₃ 3 1.4 1.5 1.5 0.2 3 0.1 Al₂O₃ 3 — 0.010.01 0.1 — — SiO₂ 3 2 — — 0.1 0.5 1.0 Cr₂O₃ 0.95 0.04 — — 0.1 — — MnO —— — — 0.1 0.5 — Mn₂O₃ — 0.6 — — 0.1 — — MgO 0.05 — — — 0.1 1 — Fe₂O₃ — —— — 0.1 0.5 — Co₂O₃ — — — — 0.1 2.5 — Co₃O₄ 2.5 0.5 0.5 0.5 0.1 — — NiO— 0.96 0.2 0.2 0.1 3 — SnO₂ — — — — 0.1 — — Li₂CO₃ — — 0.1 0.1 0.1 1.50.9 LiBiO₃ — — — — 0.1 — 1 CaO — — — — 0.1 0.5 — BreakdownField >1800 >1800 >1850 — — — — (V/mm) Nonlinearity >75 >80 >75 — — — —coefficient (α)

In the specification and claims, reference will be made to a number ofterms have the following meanings. The singular forms “a”, “an” and“the” include plural referents unless the context clearly dictatesotherwise. Approximating language, as used herein throughout thespecification and claims, may be applied to modify any quantitativerepresentation that could permissibly vary without resulting in a changein the basic function to which it is related. Accordingly, a valuemodified by a term such as “about” is not to be limited to the precisevalue specified. In some instances, the approximating language maycorrespond to the precision of an instrument for measuring the value.Similarly, “free” may be used in combination with a term, and mayinclude an insubstantial number, or trace amounts, while still beingconsidered free of the modified term.

As used herein, the terms “may” and “may be” indicate a possibility ofan occurrence within a set of circumstances; a possession of a specifiedproperty, characteristic or function; and/or qualify another verb byexpressing one or more of an ability, capability, or possibilityassociated with the qualified verb. Accordingly, usage of “may” and “maybe” indicates that a modified term is apparently appropriate, capable,or suitable for an indicated capacity, function, or usage, while takinginto account that in some circumstances the modified term may sometimesnot be appropriate, capable, or suitable. For example, in somecircumstances an event or capacity can be expected, while in othercircumstances the event or capacity cannot occur—this distinction iscaptured by the terms “may” and “may be”.

Reactants and components referred to by chemical name or formula in thespecification or claims hereof, whether referred to in the singular orplural, may be identified as they exist prior to coming into contactwith another substance referred to by chemical name or chemical type(e.g., another reactant or a solvent). Preliminary and/or transitionalchemical changes, transformations, or reactions, if any, that take placein the resulting mixture, solution, or reaction medium may be identifiedas intermediate species, master batches, and the like, and may haveutility distinct from the utility of the reaction product or finalmaterial. Other subsequent changes, transformations, or reactions mayresult from bringing the specified reactants and/or components togetherunder the conditions called for pursuant to this disclosure. In these,other subsequent changes, transformations, or reactions the reactants,ingredients, or the components to be brought together may identify orindicate the reaction product or final material.

The embodiments described herein are examples of articles, compositions,and methods having elements corresponding to the elements of theinvention recited in the claims. This written description may enablethose of ordinary skill in the art to make and use embodiments havingalternative elements that likewise correspond to the elements of theinvention recited in the claims. The scope of the invention thusincludes articles, compositions and methods that do not differ from theliteral language of the claims, and further includes other articles,compositions and methods with insubstantial differences from the literallanguage of the claims. While only certain features and embodiments havebeen illustrated and described herein, many modifications and changesmay occur to one of ordinary skill in the relevant art. The appendedclaims cover all such modifications and changes.

1. A method, comprising: contacting a transition metal oxide, asintering additive, and a grain growth inhibitor additive to form amixture, wherein the transition metal oxide comprises particles thathave an average diameter less than about 1 micrometer; and sintering themixture at a temperature profile that is sufficiently high that asintered mass is formed from the mixture, and the thermal profile isless than about 1050 degrees Celsius.
 2. The method of claim 1, whereintreating the mixture comprises exposure to a sinter temperature that isin a range from about 700 degrees Celsius to about 1050 degrees Celsius.3. The method of claim 1, further comprising calcining the transitionmetal oxide and the sintering additive together before forming themixture, and the calcining comprises heating to a temperature of about400 degress Celsius.
 4. The method of claim 1, further comprisingcontacting the sintered mass with an electrically conductive material,and forming a electrical connection therebetween.
 5. The method asdefined in claim 4, wherein the electrically conductive material has amelting point that is less than 1050 degrees Celsius, and forming theelectrical connection comprises forming the electrically conductivematerial into an electrode.
 6. The method as defined in claim 1, furthercomprising selecting the sintering additive to comprise one or more oflithium, antimony, bismuth, cobalt, manganese, or silicon.
 7. The methodas defined in claim 6, further comprising selecting the sinteringadditive to comprise one or both of LiBiO₃, or Li₂CO₃.
 8. The method asdefined in claim 1, further comprising selecting the transition metaloxide to be present in an amount that is greater than about 80 percentby weight, based on the total weight of the the sintered mass.
 9. Themethod as defined in claim 1, further comprising selecting the amount ofthe sintering additive to be less than about 15 percent by weight, basedon the total weight of the the sintered mass.
 10. The method as definedin claim 1, further comprising selecting the grain growth inhibitoradditive to comprise one or more of SiO₂, Sb₂O₃, CaO, Al₂O₃, MgO, orFe₂O₃.
 11. The method as defined in claim 1, further comprisingselecting the grain growth inhibitor additive to be present in an amountthat is less than about 10 percent by weight, based on the total weightof the the sintered mass.
 12. The method as defined in claim 1, furthercomprising adding a a grain boundary additive to the mixture prior tosintering.
 13. The method as defined in claim 12, further comprisingselecting the grain boundary additive to comprise Co₃O₄, Cr₂O₃, Bi₂O₃,Pr₂O₃, NiO, or SnO₂.
 14. The method as defined in claim 12, furthercomprising selecting the amount of the grain boundary additive to beless than about 10 percent by weight, based on the total weight of thethe sintered mass.
 15. The method as defined in claim 1, wherein thesintered mass comprises a plurality of cores, and the temperatureprofile and a an average particle size of the transition metal oxide areselected to form an average distance from one core to an adjacent corein the plurality of cores is less than about 1 micrometer aftersintering.
 16. The method as defined in claim 15, wherein the averagediameter of the cores is less than about 1 micrometer.
 17. The method asdefined in claim 15, wherein the cores are separated from each other bya grain boundary layer, and a mean value for a thickness of the grainboundary layer is less than 50 nanometers.
 18. The method as defined inclaim 17, wherein the cores each define a grain boundary in the sinteredmass, and an average distance from a grain boundary of one core to agrain boundary of an adjacent core in the sintered mass is less thanabout 1 micrometer.
 19. The method as defined in claim 12, whereinselecting the grain boundary additive results in the average thicknessof the grain boundary layer being less than about 400 nanometer.
 20. Themethod as defined in claim 1, further comprising subjecting the sinteredmass to an electrical potential, and the sintered mass exhibits adielectric strength or breakdown field of greater than about 0.5 kV/mm.21. The method as defined in claim 20, wherein the sintered mass has anon-linearity coefficient (α) of greater than about
 25. 22. The methodas defined in claim 1, further comprising forming the sintered mass witha homogenous microstructure.
 23. A method, comprising: contacting atransition metal oxide with a sintering additive to form a premix,wherein the transition metal oxide comprises particles that have anaverage diameter less than about 1 micrometer; calcining the premix,wherein the calcining comprises heating to a temperature of about 400degress Celsius to provide a calcined mass; contacting the calcined masswith a grain growth inhibitor additive to form a mixture; and sinteringthe mixture at a temperature profile that is sufficiently high that asintered mass is formed from the mixture, and the thermal profile isless than about 1050 degrees Celsius.
 24. A method, comprising:contacting a transition metal oxide, a sintering additive, and a graingrowth inhibitor additive to form a mixture, wherein the transitionmetal oxide comprises particles that have an average diameter less thanabout 1 micrometer; and sintering the mixture at less than about 1050degrees Celsius to form a sintered mass, and while sintering forming anelectrode from an electrically conductive material.